Multi-strand stressing jack

ABSTRACT

A multi-wire or strand stressing jack having an annular stressing cylinder arranged within a cylindrical outer casing and a stressing head attached to the leading end of said stressing cylinder, said stressing cylinder comprising a main plate accommodating gripping devices, a leading plate and a trailing plate operatively associated with said main plate, and means for moving said gripping devices into and out of operative positions. The jack also includes automatic lock-off means.

United States Patent Davison et a1.

[ Mar. 4, 1975 MULTI-STRAND STRESSING JACK Inventors: John Davison, Ash Vale; Hugh Jeremy Willis Edwards, Cobham, both of England Assignee: CCL Systems Limited, Surbiton,

Surrey, England Filed: Nov. 27, 1973 Appl. No.: 419,389

Related US. Application Data Continuation of Ser. No. 236,858, March 22, 1972, abandoned.

Foreign Application Priority Data Apr. 1, 1971 Great Britain 8391/71 11.8. CI. 73/143, 254/29 A Int. Cl. G011 5/04 Field of Search 73/955, 143; 254/29 A [56] References Cited UNITED STATES PATENTS 3,436,962 4/1969 Laws 73/143 3,762,686 10/1973 Gomez 254/29 A 3,811,653 5/1974 Persicke 254/29 A Primary E.\'amine1'Je1'ry W. Myracle Attorney, Agent, 01' Firm-Baldwin, Wight & Brown [57] ABSTRACT A multi-wire or strand stressing jack having an annular stressing cylinder arranged within a cylindrical outer casing and a stressing head attached to the leading end of said stressing cylinder, said stressing cylinder comprising a main plate accommodating gripping devices, a leading plate and a trailing plate operatively associated with said main plate, and means for moving said gripping devices into and out of operative positions. The jack also includes automatic lock-off means.

13 Claims, 4 Drawing Figures PATENTED 195 SHEET 2 BF 4 1 MULTl-STRANDSTRESSING' JACK This is a continuation of application Ser. No. 236,858 now abandoned filed Mar. 22, 1972.

This invention relates to multi-wire or multi-strand hydraulically operated stressing jacks for the tendons of pre-stressed concrete structures.

It is among the objects of the present invention to provide a stressing jack having improved features which result in automatic operation thereof and which lead to greater efficiency, and a reduction in the time taken to stress a tendon.

According to the present invention, there is provided a multi-wire or strand stressing jack which comprises a main cylindrical casing adapted, in use, to bear directly or indirectly against the face of a concrete structure, a pulling or stressing cylinder mounted within the main casing and adapted to be moved in an axial direction relative to the main casing, a stressing head secured to and movable with the stressing cylinder, and lock-off means operatively associated with the main casing to co-act with the stressing head, wherein the stressing head comprises a main plate secured to the stressing cylinder and formed with a plurality of spaced holes each accommodating a gripping devices, a trailing plate spaced from and movable relative to the main plate, said trailing plate having means for centralizing the gripping devices, a leading plate spaced from and movable relative to the main plate, said leading plate having means for moving the gripping devices into an inoperative position and having means operatively associated there with for urging the gripping devices into an operative position, and means for retaining the trailing and leading plates in axial relationship with the main plate.

Preferably, the means for centralizing the gripping devices comprise a compression spring co-acting with each of said gripping devices, the means for urging the gripping devices into an operative position comprise compression springs arranged between the main and leading plates, the latter being coupled to the trailing plate by bolts, and the means for moving the gripping devices into an inoperative position, comprise tubular elements attached to the leading plate and associated with each of said gripping devices.

According to the invention furthermore, the lock-off means comprise a tubular cylinder attached to one end of which is a stepped thrust head adapted, in use, to engage anchoring wedge devices provided in a concrete structure to anchor the wires or strands of the tendon in position therein.

According to a further feature of the invention, the thrust head incorporates threading tubes which are slidably mounted therein to facilitate positioning of the wires or strands in the stressing head.

According to a still further feature of the invention, the jack includes, as an integral part thereof, load measuring means. Thus, the main cylindrical casing may be strain gauged and utilized, in use, to transmit signals to a load measuring device mounted on, or associated with, the jack. v

The invention is illustrated by way of example in the accompanying drawings in which,

FIG. I is a longitudinal section, partly in elevation, of part of a stressing jack according to the invention, fig ure being in two parts labelled respectively FIG. 1(a) and FIG. 1(b). The two parts are to be read together as joined along the chain lines.

FIG. 2 is a part and elevation taken in the direction of the arrow 2 of FIG. 1 with the bearing ring removed, and

FIG. 3 is a part section on line 33 of FIG. 2.

Referring to the drawings, the stressing jack comprises a main cylinder 1 having fixed thereto at its leading end, by means of bolts 2, an extension cylinder 3. A nose structure 4 in the form of an annulus is fixed by bolts 5 to the leading end of the cylinder 3 and is arranged, in use, to be placed over the wires or strands of the tendon to be stressed and to bear directly, or via a bearing ring 6, against the concrete structure housing the tendon. The trailing end of the main cylinder I has associated therewith an annular piston 7 which has fixed thereto an outer cylinder 8 and an inner or stressing cylinder 9 both of which cylinders extend towards the leading end of the jack and overlap the main cylinder 1. Sealing rings 10 are provided between the piston 7 and the outer cylinder 8, and between the piston 7 and the stressing cylinder 9. A further sealing ring 11 is arranged between the main cylinder 1 and the stress ing cylinder 9. In addition, a double acting sealing ring 12 is positioned between the outer cylinder 8 and a seal piston 13 fixed on the trailing end of the main cylinder 1.

A fluid port 14 is arranged in the piston 7 to communicate with a pressure chamber 15 so that when hydraulic fluid under pressure is pumped into the port 14 the main ram, consisting of the piston 7 and the cylinders 8 and 9, is extended. A fluid port 16 is provided in the outer cylinder 8 to communicate with a pressure chamber 17 so that when hydraulic fluid is pumped into the port 16 the main ram will be caused to retract.

The innermost or leadng end of the inner or stressing cylinder 9 has fixed thereto a stressing head generally indicated at 18. The stressing head 18 comprises a main plate 19 which is secured to the inner or stressing cylinder 9 by means ofa flange 20 formed on the end of the latter and a retaining tube 21 secured at one end to the inner or stressing cylinder 9 by bolts 22, a trailing plate 23 and a leading plate 24. The main plate 19 is formed with a plurality of spaced tapered. holes 25 in each of which is received a gripping device 26 in the form of two or more wedge-shaped gripping elements.

The trailing plate 23 is formed with stepped holes 27, which correspond in position and number to the holes 25, and which receive flanged pressure-applying tubes 28 which are adapted to bear on the ends of their corresponding gripping devices 26. The tubes 28 have operatively associated therewith compression springs 29 which serve to bias the gripping devices 26 into an operative position where, in use, they engage and grip the wires or strands, which are passed therethrough, of a tendon to be stressed.

The leading plate 24 is formed with a plurality of stepped holes 30, corresponding in number and position to the holes in the main and trailing plates, each hole having received therein a gripping device release tube 31 which extends into its corresponding hole in the main plate 19. Some or all of the tubes 31 have associated therewith compression springs 32, the springs 32 serving to bias the main and leading plates into a position where they are in spaced relationship with one another, and also, in use, to urge the gripping devices 26 in an operative position as will hereinafter be explained.

Normally, the plates 19, 23 and 24 are maintained in predetermined spaced relationship by means of a plurality of bolts 33. These bolts are fixed to the leading plate 24 by screw-threading, and are passed through oversized holes in the plates 19 and 23 so that the latter are axially slidable relative thereto.

Arranged between the main cylinder 1 and the inner cylinder 9, is a lock-off head consisting of an annular lock-off cylinder 34 the leading end of which has fixed thereto, by bolts 35, a thrust plate 36 which, in turn, has secured thereto by bolts 37 thrust head plates 38. The plates 36 and 38 are formed with a plurality of axial passages 39 corresponding in number and position to the holes in the members of the stressing head 18. Each of the axial passages 39 has slidably received therein a threading tube 40 having an enlarged head portion 41.

As can be seen from FIG. 1 of the drawings, the lockoff cylinder 34 is formed to present, with the main cylinder l and an annular insert 42, a pressure chamber 43 which communicates with a fluid port 44. The lockoff cylinder 34 also presents, with the main cylinder 1 and the extension cylinder 3, a pressure chamber 45 which communicates with a fluid port 46. Thus, fluid pressure applied through the port 44 will cause the lock-off cylinder to extend, and fluid pressure applied through the port 46 will cause it to retract.

In use, in order to stress a tendon having a plurality or wires or strands, the jack, in its retracted position as shown in the drawing, is offered up to the tension and the threading tubes 40, 41 are pulled axially out of the lock-off head and placed over the ends of the wires or strands. The jack is then moved towards the concrete structure so that the wires or strands, guided by the threading tubes, are moved into the stressing head 18 and are positioned automatically in the wedge devices 26. During this operation, the heads 41 of the tubes 40 will engage the anchorage wedges of the structure and, after the threading tubes are pushed back into the jack, will hold them in their correct positions in the structure.

Hydraulic fluid under pressure is applied to the port 14 in order to extend the jack so that the wedgedevices automatically grip the wires or strands and so that tension is applied to the wires or strands via the inner cylinder 9 and the stressing head 18.

It will be appreciated that, when the jack is extended, the leading plate 24 will be moved, under the action of the springs 32, further away from the main plate 19. This, via the bolts 33, will move the trailing plate 23 towards the main plate 19 and, in doing so, will compress the springs 29 so that flanges of the tubes 28 are in engagement with the trailing plate 23 whereby pressure will be applied to the wedge devices 26 by the springs 32. It will therefore be seen that, during operation, the main pressure applied to the wedge devices 26 eminates from the springs 32, the springs 29 serving only to initially hold the wedge devices in position, and to take up any misalignment in the positions of the back faces of the wedges which may occur due to machining tolerances in the tapered holes 25 or on the wedges themselves.

When the required stress has been applied to the wires or strands, the lock-off cylinder 34 is extended to cause the threading tubes 40, via the lock-off head, to force home the anchorage wedges in the concrete structure and to anchor the wires or strands in the structure, after which the lock-off head is retracted. Hydraulic fluid is then applied to the port 16 to retract the jack.

As the stressing head 18 nears its fully retracted position, the leading plate 24 will engage the thrust plate 36 and will be caused, aganst the action of springs 32, to move towards the main plate 19 and thereby cause the release tubes 31 to engage the wedge devices 26 and to open them against the action of the springs 29. Thus, when the stressing head 18 reaches its fully retracted position, the wedgedevices 26 will be held in an open position. In this position, the jack can be removed. Alternatively, the jack can be operated from the new position of the wedge devices on the wires or strands to apply further tension to the wires or strands.

Instead of utilizing the usual load cells, which increases the length of the jack, for measuring the stress applied, a part of the jack is strain gauged and used for this purpose. Thus, the main cylinder 1 is calibrated and has attached thereto within an annular space 47, strain gauges 48 which are adapted to transmit signals, via a plug and socket connection 49, to a remote indicating meter. The information transmitted can then be passed on to a visual control panel and/or to a recording device. As can be seen in FIG. 1, the annular space 47 is enclosed by an annular cover plate 50 which is held in position by means of a flange 51 arranged between the main cylinder 1 and the extension cylinder 3. The cover plate 50' is formed with an aperture 52 through which leads 53 extend to the plug 49, the latter being supported on a bracket 54 which also acts as a cover plate for the aperture 52.

In addition to the aforementioned strain gauge means for measuring the load applied to the tendon being stressed, means are provided for measuring the physical extent of the tension applied to the tendon. These means comprise a calibrated scale 55 which is fixed to, or marked on, the outer surface of the annular cover plate 50 to co-act with a sliding marker element 56 attached to and movable with the outer cylinder 8.

It will be appreciated that the structure hereinbefore described, apart from presenting a fully automatic jack, has the advantage that it can be used with tendons made up of different numbers of strands. In this respect, although the example described shows an arrangement for a tendon having twelve strands, tendons having more or less strands can be handled by replacing the stressing head 18 and the lock-off cylinder head. To

do this, all that is necessary is to remove the bolts 22 in order to remove the retaining tube 21 and thereafter the stressing head can be removed and replaced. At the same time, the lock-off cylinder head, consisting of the thrust plate 36 and the head plates 38 with the tubes 40, can be replaced by unscrewing the bolts 5 to remove the nose structure 4, and thereafter unscrewing the bolts 35. As can be seen, this operation does not in any way affect the other working parts of the jack.

We claim:

1. A multi-wire or strand stressing jack which comprises a main cylinder casing adapted, in use, to bear directly or indirectly against the face of a concrete structure housing a pluality of wires or strands, a stressing cylinder mounted within said main casing and being movable under fluid pressure in an axial direction relative to said main casing, means for subjecting said stressing cylinder to fluid pressure, a stressing head secured to and movable with said stressing cylinder, and

lock-off means mounted within said main casing to coact with said stressing head, wherein said stressing head comprises a main plate secured to said stressing cylinder and formed with a plurality of spaced holes each accommodating a gripping device, a trailing plate mounted at one side of and being spaced from and movable relative to said main plate, a leading plate mounted on the opposite side of and being spaced from and movable relative to said main plate, said leading plate having means operatively connected therewith to urge said gripping devices into an operative position as well as means for moving said gripping devices into an inoperative position, and other means for retaining said trailing and said leading plates in axial relationship wit said main plate.

2. A stressing jack as claimed in claim 1, in which means are connected to said trailing plate for centralizing said gripping devices, said centralizing means comprising, in respect of each of said gripping devices, a compression spring operatively connected to a flanged pressure applying tube which is slidably mounted in said trailing plate to engage its co-acting gripping device.

3. A stressing jack as claimed in claim 1, in which said means for maintaining said trailing and said leading plates in position relative to said main plate, comprise a plurality of bolts fixed at one end to said leading plate and extending loosely through said main and said trailing plates.

4. A stressing jack as claimed in claim 3, in which said means for urging said gripping devices into an operative position comprise a plurality of compression springs positioned between said main and said leading plates, said springs serving to move said leading plate away from said main plate and, via said bolts, to move said trailing plate towards said main plate.

5. A stressing jack as claimed in claim 4, in which said means connected with said leading plate to move said gripping devices into an inoperative position, comprise release tubes fixed in said leading plate to extend into said holes in said main plate.

6. A stressing jack as claimed in claim 1, including a retaining tube positioned within and extending the length of said stressing cylinder, and in which said stressing cylinder is formed at its leading end with an inwardly directed flange and said stressing head is fixed to said stressing cylinder by securing said main plate between said flange and said retaining tube.

7. A stressing jack as claimed in claim 1, in which said lock-off means comprise a lock-off cylinder positioned between said main and said stressing cylinders, and a stepped head attached to the leading end of said lock-off cylinder, said stepped head being formed with axial holes for the passage of said wires or strands and said holes corresponding in number and position to said holes in said main plate.

8. A stressing jack as claimed in claim 7, in which each of said holes in said lock-off head has a threading tube slidably received therein.

9. A stressing jack as claimed in claim 1, including strain gauge means attached to said main casing and being responsive to straining of said main casing, and means for transmitting from said. strain gauge means and to a remote location signals indicating the stress applied to said wires or strands.

10. A stressing jack as claimed in claim 1 including an indicator movable with a movable part of said jack, and means to measure the physical extent of tension applied to said wires or strands, said means comprising a calibrated scale on a fixed part of the jack and being cooperable with said indicator.

11. A muIti-wire or strand stressing jack which comprises a main cylinder casing adapted, in use, to bear directly or indirectly against the face of a concrete structure housing a plurality of wires or strands, a stressing member mounted within said main casing and being movable under fluid pressure in an axial direction relative to said main casing, means for subjecting said stressing member to fluid pressure, a stressing head secured to and movable with said stressing member, and hydraulically actuated lock-off means mounted within said main casing to co-act operationally with said stressing head, wherein said stressing head comprises a main plate attached to said stressing member and formed with a plurality of spaced holes each accommodating a gripping device, a trailing plate mounted at one side of and being spaced from said main plate, a leading plate mounted on the opposite side of and being spaced from said main plate, said leading plate having means operatively connected therewith to urge said gripping devices into an inoperative position, other means urging said gripping devices into an operative position, and means for effecting relative movement, in the direction of the longitudinal axis of said main cylinder casing, of said plates for causing said gripping devices to be urged into their operative and inoperative positions.

12. A stressing jack as claimed in claim 11, in which said main cylinder casing is adapted, in use, to bear against a bearing plate positioned against said face of said concrete structure.

13. A stressing jack as claimed in claim 11, in which said relative movement between said plates is effected by turning movement of said stressing member. 

1. A multi-wire or strand stressing jack which comprises a main cylinder casing adapted, in use, to bear directly or indirectly against the face of a concrete structure housing a pluality of wires or strands, a stressing cylinder mounted within said main casing and being movable under fluid pressure in an axial direction relative to said main casing, means for subjecting said stressing cylinder to fluid pressure, a stressing head secured to and movable with said stressing cylinder, and lock-off means mounted within said main casing to co-act with said stressing head, wherein said stressing head comprises a main plate secured to said stressing cylinder and formed with a plurality of spaced holes each accommodating a gripping device, a trailing plate mounted at one side of and being spaced from and movable relative to said main plate, a leading plate mounted on the opposite side of and being spaced from and movable relative to said main plate, said leading plate having means operatively connected therewith to urge said gripping devices into an operative position as well as means for moving said gripping devices into an inoperative position, and other means for retaining said trailing and said leading plates in axial relationship with said main plate.
 2. A stressing jack as claimed in claim 1, in which means are connected to said trailing plate for centralizing said gripping devices, said centralizing means comprising, in respect of each of said gripping devices, a compression spring operatively connected to a flanged pressure applying tube which is slidably mounted in said trailing plate to engage its co-acting gripping device.
 3. A stressing jack as claimed in claim 1, in which said means for maintaining said trailing and said leading plates in position relative to said main plate, comprise a plurality of bolts fixed at one end to said leading plate and extending loosely through said main and said trailing plates.
 4. A stressing jack as claimed in claim 3, in which said means for urging said gripping devices into an operative position comprise a plurality of compression springs positioned between said main and said leading plates, said springs serving to move said leading plate away from said main plate and, via said bolts, to move said trailing plate towards said main plate.
 5. A stressing jack as claimed in claim 4, in which said means connected with said leading plate to move said gripping devices into an inoperative position, comprise release tubes fixed in said leading plate to extend into said holes in said main plate.
 6. A stressing jack as claimed in claim 1, including a retaining tube positioned within and extending the length of said stressing cylinder, and in which said stressing cylinder is formed at its leading end with an inwardly directed flange and said stressing head is fixed to said stressing cylinder by securing said main plate between said flange and said retaining tube.
 7. A stressing jack as claimed in claim 1, in which said lock-off means comprise a lock-off cylinder positioned between said main and said stressing cylinders, and a stepped head attached to the leading end of said lock-off cylinder, said stepped head being formed with axial holes for the passage of said wires or strands and said holes corresponding in number and position to said holes in said main plate.
 8. A stressing jack as claimed in claim 7, in which each of said holes in said lock-off head has a threading tube slidably received therein.
 9. A stressing jack as claimed in claim 1, including strain gauge means attached to said main casing and being responsive to straining of said main casing, and means for transmitting from said strain gauge means and to a remote location signals indicating the stress applied to said wires or strands.
 10. A stressing jack as claimed in claim 1 including an indicator movable with a movable part of said jack, and means to measure the physical extent of tension applied to said wires or strands, said means comprising a calibrated scale on a fixed part of the jack and being cooperable with said indicator.
 11. A multi-wire or strand stressing jack which comprises a main cylinder casing adapted, in use, to bear directly or indirectly against the face of a concrete structure housing a plurality of wires or strands, a stressing member mounted within said main casing and being movable under fluid pressure in an axial direction relative to said main casing, means for subjecting said stressing member to fluid pressure, a stressing head secured to and movable with said stressing member, and hydraulically actuated lock-off means mounted within said main casing to co-act operationally with said stressing head, wherein said stressing head comprises a main plate attached to said stressing member and formed with a plurality of spaced holes each accommodating a gripping device, a trailing plate mounted at one side of and being spaced from said main plate, a leading plate mounted on the opposite side of and being spaced from said main plate, said leading plate having means operatively connected therewith to urge said gripping devices into an inoperative position, other means urging said gripping devices into an operative position, and means for effecting relative movement, in the direction of the longitudinal axis of said main cylinder casing, of said plates for causing said gripping devices to be urged into their operative and inoperative positions.
 12. A stressing jack as claimed in claim 11, in which said main cylinder casing is adapted, in use, to bear against a bearing plate positioned against said face of said concrete structuRe.
 13. A stressing jack as claimed in claim 11, in which said relative movement between said plates is effected by turning movement of said stressing member. 